1. Field of the Invention
The present invention relates to a control device for an induction motor and, more particularly, to a control device with which it is possible to keep the efficiency of an induction motor at the maximum value irrespective of its load condition.
2. Description of the Prior Art
FIG. 1 is a block diagram depicting a conventional control device disclosed, for example, in Japanese Pat. Appln. Laid-Open Gazette No. 89493/87. Reference numeral 1a denotes an inverter, 2 an induction motor, 3 a current sensor, 21 a power rectifying part, 22 a capacitor, 23 a power inverting part, 24 a rectifier, 25 a low-pass filter, 26 an A/D converter, 27 a control circuit formed by a microcomputer, and 28 a PWM (Power Width Modulation) circuit. The inverter 1a provided with the capacitor 22 is combined with the PWM circuit 28 to constitute a known voltage shift PWM inverter which functions to supply a variable voltage variable frequency, three-phase AC voltage to the induction motor 2.
FIG. 2 is a diagram explanatory of the principle of operation of a high-efficiency control scheme in the conventional induction motor control. The amplitudes of an input current (primary current) and an input voltage (primary voltage) of the induction motor bear such a relationship as indicated by a characteristic curve A in FIG. 2 when the on-load torque is constant. With an input voltage raised higher than it needs to be, an exciting current increases, which causes an increase in a primary copper loss or iron loss, hence inevitably impairing the efficiency of the induction motor. Conversely, when the input voltage is lower than it needs to be, slip power increases and a secondary current increases, which causes an increase in primary and secondary copper losses, also decreasing the efficiency of the induction motor. Since the primary current of the induction motor is expressed as the sum of vectors of the exciting current and the secondary current, the loss becomes minimum at the point where the primary current becomes minimum (the point B in FIG. 2). Accordingly, the induction motor can be driven with the maximum efficiency by controlling the amplitude of the primary voltage to minimize the amplitude of the primary current.
Next, the operation of the prior art example will be described.
The primary current detected by the current sensor 3 is applied to the low-pass filter 25, from which is provided the average value of the primary current. The average value is provided via the A/D converter 26 to the control circuit 27. Based on the above-mentioned principle of operation, the control circuit 27 calculates a primary voltage command value which decreases the average value of the primary current, and provides the calculated value to the PWM circuit 28. As a result, the voltage shift PWM inverter formed by the PWM circuit 28 and the inverter 1a supplies the induction motor 2 with a primary voltage that matches the primary voltage command value provided from the control circuit 27.
Since the conventional induction motor control device detects, as described above, the primary current of the induction motor and controls the amplitude of the primary voltage in a manner to minimize the amplitude of the primary current, information necessary for control is the primary current alone, hence permitting control with an inexpensive device configuration. In the case of variable speed driving of the induction motor, however, the amplitude of the primary voltage varies with changes in a frequency command value of the voltage shift PWM inverter, it is impossible to control the amplitude of the primary voltage in such a manner as to minimize the amplitude of the primary current. On this account, the conventional control device is incapable of driving the induction motor with the maximum efficiency during variable speed operation.
Furthermore, during constant speed driving, too, when the induction motor-generated torque goes below the on-load torque due to too rapid changes in the amplitude of the primary voltage and the rotational speed of the induction motor begins to decrease, induced voltage also drops in proportion to the speed of the motor. In consequence, further reduction of the amplitude of the primary voltage leads to a decrease in the amplitude of the primary current, giving rise to a problem that the induction motor stops at the worst. To avoid this, it is necessary to make the amplitude variation of the primary voltage gentle, but in a particular use of the induction motor that involves frequent changes in the on-load torque during fixed speed driving, so it is impossible to achieve maximum efficiency driving that follows in variations in the on-load torque well.